Lung Function Testing: How Breathing Tests Diagnose and Monitor Asthma

When you visit a pulmonologist for breathing problems, one of the most important steps is lung function testing — a group of painless, non-invasive tests that measure exactly how well your lungs are working. These tests go far beyond what a stethoscope can tell your doctor. They provide objective, measurable data that reveals whether your airways are narrowed, how much airflow is restricted, and whether asthma or another condition is causing your symptoms.

At Advanced Asthma Clinic in Plantation, FL, Dr. Frank Hull uses comprehensive lung function testing as the foundation of every asthma evaluation. With over 20 years of pulmonary research experience, Dr. Hull relies on these measurements not just to diagnose asthma — but to classify its severity, guide treatment choices, and track your response to therapy over time.

What Is Lung Function Testing?

Lung function testing (also called pulmonary function testing or PFTs) encompasses several different breathing tests, each measuring a specific aspect of how your lungs work. Think of it as a comprehensive physical exam for your respiratory system. Just as blood tests reveal information invisible to the naked eye, lung function tests quantify airflow, lung capacity, and airway inflammation with precision that symptoms alone cannot provide.

These tests are essential because asthma symptoms like wheezing, shortness of breath, and coughing can also be caused by other conditions — from vocal cord dysfunction to COPD, heart disease, or anxiety. Objective lung function data helps your doctor distinguish between these conditions and build an accurate diagnosis.

Types of Lung Function Tests

Spirometry: The Foundation of Asthma Diagnosis

Spirometry is the most common and most important lung function test for asthma. You breathe into a mouthpiece connected to a device called a spirometer, which measures how much air you can exhale and how quickly you can move it. The test takes about 15-30 minutes and is completely painless.

During spirometry, you will:

• Breathe normally for several breaths to establish a baseline
• Take the deepest breath possible, then blow out as hard and as fast as you can until your lungs are completely empty
• Repeat the maneuver at least three times to ensure consistent, reliable results

The key measurements from spirometry include:

Bronchodilator Reversibility Testing

This test is one of the most valuable tools for distinguishing asthma from other obstructive lung diseases. After completing baseline spirometry, you inhale a short-acting bronchodilator (typically albuterol), wait 15-20 minutes, and then repeat the spirometry measurements.

If your FEV1 improves by at least 12% and 200 mL after the bronchodilator, this is considered a "positive" reversibility response — strong evidence of asthma. This reversibility is a hallmark feature that separates asthma from conditions like COPD, where airflow obstruction is largely fixed and irreversible.

FeNO Testing: Measuring Airway Inflammation

Fractional Exhaled Nitric Oxide (FeNO) testing is a breakthrough technology that measures eosinophilic airway inflammation — the type of inflammation that drives most allergic and moderate-to-severe asthma. While spirometry measures the consequences of airway disease (narrowing and obstruction), FeNO measures the underlying inflammation that causes it.

The test is remarkably simple: you take a slow, steady breath out into a handheld device for about 10 seconds. Results are available within minutes.

How to interpret FeNO levels:

• Below 25 ppb (adults): Low — eosinophilic inflammation is unlikely to be the primary driver of symptoms
• 25-50 ppb: Intermediate — inflammation may be contributing; clinical context determines significance
• Above 50 ppb: High — significant eosinophilic inflammation present; strongly suggests asthma and likely good response to corticosteroids or biologic therapies

FeNO testing is especially valuable for determining whether a patient is a candidate for biologic medications like Dupixent, Nucala, Fasenra, or Tezspire — advanced treatments that target specific inflammatory pathways. Dr. Hull uses FeNO testing routinely to guide these treatment decisions and monitor response to therapy.

Peak Flow Monitoring

A peak flow meter is a simple, portable device you can use at home to track your lung function daily. By blowing into the device each morning and evening, you generate a peak expiratory flow (PEF) reading that shows how open your airways are at that moment.

Peak flow monitoring is particularly useful for:

• Detecting early warning signs: A declining peak flow often precedes noticeable symptoms by hours or days, giving you time to adjust treatment before a flare-up develops
• Identifying triggers: Tracking peak flow alongside activities and weather changes can reveal patterns in what triggers your asthma
• Guiding your asthma action plan: Your personal best peak flow number helps define the green, yellow, and red zones that tell you when to step up treatment or seek emergency care
• Monitoring occupational exposures: Comparing workplace and home readings can help diagnose occupational asthma

Flow-Volume Loop Analysis

A flow-volume loop is a graphical representation of your spirometry data that plots airflow speed against lung volume during both inhalation and exhalation. The shape of this curve provides diagnostic information beyond what the raw numbers alone reveal.

In asthma, the expiratory portion of the loop shows a characteristic "scooped out" or concave shape, reflecting airflow limitation that worsens as you exhale. This pattern differs distinctly from vocal cord dysfunction, which produces a flattened inspiratory loop, or from fixed upper airway obstruction, which flattens both the inspiratory and expiratory portions.

When Do You Need Lung Function Testing?

Lung function testing is recommended in several situations:

• Initial diagnosis: If you have symptoms suggestive of asthma — recurrent wheezing, chest tightness, shortness of breath, or chronic cough — spirometry with bronchodilator reversibility testing is the standard diagnostic approach
• Asthma not responding to treatment: If standard medications are not controlling your symptoms, repeat testing can reveal whether the diagnosis is correct or whether a different condition (like VCD or COPD) is contributing
• Before starting or changing biologic therapy: Baseline lung function helps measure how much a biologic medication improves your condition
• Pre-surgical clearance: Lung function tests assess whether your respiratory system can safely handle anesthesia and surgery
• Monitoring chronic conditions: Regular spirometry tracks disease progression and treatment response in asthma and COPD
• Unexplained shortness of breath: When the cause of breathing difficulty is unclear, comprehensive PFTs help narrow the differential diagnosis
• Clinical trial eligibility: Most asthma and COPD clinical trials require specific lung function criteria for enrollment

How to Prepare for Your Lung Function Test

Proper preparation ensures accurate, reliable results. Before your appointment:

• Medications: Stop short-acting bronchodilators (albuterol/levalbuterol) for 6-8 hours before testing. Stop long-acting bronchodilators for 24 hours. Do not stop controller medications unless specifically instructed by your doctor.
• Smoking: Avoid smoking for at least 4-6 hours before the test
• Food: Avoid heavy meals for 2 hours before testing — a full stomach can restrict lung expansion
• Exercise: Avoid vigorous physical activity for at least 30 minutes before the test
• Clothing: Wear loose, comfortable clothing that does not restrict your chest or abdomen
• Illness: If you have an active respiratory infection, contact your doctor — testing may need to be rescheduled to get accurate baseline results

Understanding Your Results

Your spirometry results are compared to predicted values based on your age, height, sex, and ethnicity. Results are reported as a percentage of predicted normal:

It's important to understand that a single spirometry result is just a snapshot. Asthma is a variable disease — your lung function may be normal on a good day and significantly reduced during a flare-up. This is why trending your results over time and combining spirometry with FeNO and clinical assessment gives the most complete picture.

Lung Function Testing for Special Populations

Children and Adolescents

Spirometry can be performed reliably in most children ages 6 and older. For younger children who cannot perform the forced breathing maneuvers, alternative techniques like impulse oscillometry may be used. Early lung function assessment is critical in pediatric asthma to establish baseline values and ensure appropriate treatment from the start.

Athletes

Athletes with exercise-related breathing problems may have normal resting spirometry but show significant airflow limitation after exercise challenge testing. Exercise bronchoprovocation testing — spirometry performed before and at intervals after vigorous exercise — can unmask exercise-induced bronchoconstriction (EIB) that resting tests miss.

Older Adults

In older patients, distinguishing between asthma and COPD — or identifying asthma-COPD overlap — requires careful interpretation of lung function data. Bronchodilator reversibility testing and FeNO are particularly helpful in this population, where symptoms of both conditions frequently coexist.

Advanced Asthma Clinic: Expert Pulmonary Diagnostics

Dr. Frank Hull at Advanced Asthma Clinic provides comprehensive lung function testing as part of every asthma evaluation. With over 20 years of pulmonary research experience and access to the latest diagnostic technology — including spirometry, bronchodilator reversibility testing, FeNO measurement, and flow-volume loop analysis — Dr. Hull can accurately diagnose your condition, determine its severity, and develop a personalized treatment plan based on objective data, not guesswork.

Our Plantation, FL office also serves as a clinical research site, which means patients benefit from the same rigorous testing standards used in clinical trials — ensuring the highest accuracy and reliability in every test we perform.